The invention relates to a fluidic microsystem and to a method for particle manipulation, in particular for particle manipulation with high-frequency electrical fields.
It is known to manipulate suspended particles (e.g. biological cells, cell groups, cell components, macromolecules or synthetic particles in suspension solutions) in fluidic Microsystems with high-frequency electrical fields which are generated with the use of microelectrodes in channels of the microsystem (see e.g. T. Schnelle et al. in “Langmuir”, vol. 12, 1996, pp. 801-809). Touchless particle manipulation (e.g. moving, stopping, deflecting, fusing, etc.) is based on negative dielectrophoresis. It is well known to at least partly cover the microelectrodes arranged on channel walls with an electrically insulating thin layer in order to minimize undesirable interaction between the microelectrodes and the suspension medium or the particles, such as e.g. ohmic losses, electrolysis, induction of transmembrane potentials etc. (passivation of the microelectrodes).
Typically, the fluidic microsystems comprise spatial electrode arrangements. The microelectrodes are arranged at opposite, e.g. upper and lower, channel walls with typical spacing ranging from 10 μm to 100 μm (see T. Müller et al. in “Biosensors & Bioelectronics”, vol. 14, 1999, pp. 247-256). In order to achieve defined field effects, the microelectrodes have to be formed and arranged relative to each other in a particular way. In the case of spatial electrode arrangements this involves very considerable effort in adjusting the channel walls (chip planes). With typical microsystem dimensions in the cm range, the accuracy has to be better than 5 μm. Furthermore, there are problems in the production of the microsystem. Usually, production takes place with techniques used in semiconductor technology, wherein for the spatial electrode arrangement several masks are required for wafer processing. Finally, spatial electrode arrangement involving structured microelectrodes on various channel walls is associated with a problem in relation to electrical contacting. As a rule, electrical contacting needs to be carried out from the top channel wall (top chip plane) to the bottom channel wall, and needs to be led, electrically separated from said bottom channel wall, to a control connection. In particular with a view to mass use of fluidic Microsystems there is an interest in Microsystems of a simplified design and with enhanced functional safety.
It has been known to structure electrically insulating passivation layers in order to obtain a particular field shaping (see DE 198 69 117, DE 198 60 118). Structuring consists of making apertures or breakdowns into the passivation layer above an area-type electrode. Through the apertures, the electrical field can penetrate from the electrode to the channel, and can form the desired field form corresponding to the shape of the aperture. The apertures in the passivation layers are however associated with the disadvantage in that contact is established between the electrode material and the suspension liquid. There is a possibility of irreversible electrode processes occurring. For example, as a result of the field effect, particles can be drawn onto the electrodes and can block the channel. Furthermore, dissolution of the electrode material and thus contamination of the suspension liquid can occur. Up to now, this problem has been countered by the use of suspension liquids with a rather low electrolyte content. However, this has limited the scope of application of the Microsystems. Many biological particles are only able to tolerate a low electrolyte content to a limited degree for an extended period of time.
It is also known that the passivation layers on microelectrodes cause field shielding. This can for example be used in order to strengthen or weaken field gradients in the channel according to a particular spatial gradient (see e.g. T. Schnelle et al., see above, and G. Fuhr et al. in “Sensors and Materials”, vol. 7/2, 1995, pp. 131-146). However, there is a disadvantage in that the weakening influence of the passivation layer in suspension fluids with a low electrolyte content (low conductivity) is relatively weak.
It is the object of the invention to provide an improved fluidic microsystem which overcomes the disadvantages of conventional Microsystems. It is in particular the object of the invention to provide a microsystem of a simplified design, in particular simplified electrode arrangement and simplified contacting, enhanced functional safety and an expanded field of application, in particular in the manipulation of biological particles. Furthermore, it is the object of the invention to provide an improved method for the field shaping in fluidic Microsystems, in particular for dielectrophoretic manipulation of particles.